I have naturally weak nails, and this stuff really makes a difference. Follow the instructions (apply every few days). Only 4 stars because it's pretty expensive. Haven't tried using it under polish yet. Also, it's not really matte. It's not super glossy, but it is definitely visible.

Bought this for my husband who has always had trouble with his nails splitting. We started applying this to his nails once a week and the splitting and cracking have disappeared! It has even improved the ridging that he had started to notice. The matt finish looks very natural and does not draw attention to his nails.